Mechanical shield to protect magnetic core in xerographic developing apparatus



Nov. 23, 1965 G. R. MOTT ETAL MECHANICAL SHIELD T O PROTECT MAGNETIC GORE I XEROGRAPHIC DEVELOPING APPARATUS Filed Dec. 4, 1962 F/GZ INVENTORS GEORGE R. MOTT BY HAROLD E. CLARK ATTORNEY United States Patent 3,219,014 Patented Nov. 23, 1965 MECHANICAL SHIELD TO PROTECT MAG- NETIC CORE IN XEROGRAPHIC DEVELOP- ING APPARATUS George R. Mott, Rochester, and Harold E. Clark, Penfield, N.Y., assignors to Xerox Corporation, Rochester, N.Y., a corporation of New York Filed Dec. 4, 1962, Ser. No. 242,251 2 Claims. (Cl. 118-637) This invention relates to xerography and more particularly to improved apparatus for developing electrostatic images. The application is a continuation-in-part of copending application, Serial No. 163,288, filed December 29, 1961.

In xerography, it is usual to form an electrostatic latent image on a surface. One method of doing this is to charge a photoconductive insulating surface and then dissipate the charge selectively by exposure to a pattern of activating radiation as set forth, for example, in U.S. 2,297,691 to Chester F. Carlson. Other means of forming electrostatic latent images are set forth in US. 2,647,- 464 to James P. Ebert. Whether formed by these means or any other, the resulting electrostatic charge pattern is conventionally utilized by the deposition of an electroscopic material thereon through electrostatic attraction whereby there is formed a visible image of electroscopic particles corresponding to the electrostatic latent image. Alternatively, the electrostatic charge pattern may be transferred to an insulating film and the electroscopic particles deposited thereon to form the visible image. In any case this visible image, in turn, may be transferred to a second surface to form a xerographic print or may be affixed directly to the surface on which it is developed.

A process of applying the developer to the latent electrostatic image is set forth in US. 2,618,552 to E. N. Wise and involves the use of a finely-divided colored material called a toner deposited on a slightly more coarsely divided material called a carrier. This twocomponent developer is cascaded across the electrostatic image areas. To produce a positive of the electrostatic image, a toner and carrier are selected such that the toner will be charged to a polarity opposite to that of the electrostatic image, the carrier being charged to the same polarity as the electrostatic image. When a carrier particle, bearing on its surface oppositely charged particles of toner, crosses an area on the image surface having an electrostatic charge, the charge on the image surface exerts a greater attraction for the toner than the carrier and retains the toner in the charged area and separates it from the carrier particles. The carrier particles being oppositely charged and having greater momentum are not retained by the charged areas of the plate. When a toned carrier particle passes over a non-charged area of the plate, the electrostatic attraction of the carrier particles for the toner particles is sufiicient to retain the toner on the carrier preventing deposition in such areas as the carrier particles momentum carries both toner and carrier past. By this mechanism, the image is developed, or as is generally the case, it is made visible.

Another form of developer mix is composed of a toner and a ferro-magnetic carrier material controlled by means of a magnet. This forms streamers of the developer which constitute a brush-like mass and the brush is passed over the surface bearing the electrostatic latent image whereby the brush contacts the image bearing surface. Development by this means is termed magnetic brush development. The toner is triboelectrically charged and deposits on the electrostatic latent image in a manner similar to that wherein the toner and carrier mix is cascaded across the image-bearing surface. Variations on the use of ferro-magnetic developer are set forth, for example, in US. 2,846,333 to Wilson and US. 3,015,- 305 to Hall et al.

In connection with magnetic brush development, it is known in the art, as disclosed for example in Giairno patent, US. 2,890,968, that applying a bias potential to a conductive magnet is effective in altering the electrostatic field by which development of the electrostatic image takes place. By appropriately selecting the polarity and magnitude of the bias potential as explained in the Giaimo patent above, it is possible to alter the field in a manner to achieve various results including image surpression, enhancement, and even complete reversal of the development field.

Despite these known benefits, this prior technique of biasing the magnetic member suffers from many disadvantages and as such, has handicapped utility of the technique for commercial application. Among the disadvantages is the difficulty of handling the magnetic member for cleaning purposes without the inherent shock hazard present to the operator unless proper precautions are taken. At the same time, the magnetic member in order to conduct a bias potential must of necessity be restricted to an electrically conductive material excluding the use of magnetic insulators and, which for many developing applications, is undesirable as will be understood from the discussion below.

Now in accordance with the improved apparatus of the instant invention, there has been discovered that the prior disadvantages can be overcome by employing an electrically conductive shield supported about the magnet. The bias effect can be achieved by applying the bias potential to the shield instead of the magnet. Further, by forming the shield of a conductive exterior integrally supported on an insulating material, the benefits of the insulating material can be selectively utilized as to broaden the range of development application independent of the electrical properties of the magnet. Since the magnetic member does not per se support the developer, cleaning can be effected simply by removing the magnet from the shield or vice versa.

It is accordingly an object of the invention to provide novel magnetic brush developing apparatus for developing electrostatic latent images.

It is a further object of the invention to provide a novel means by which reference potentials can be applied to a magnetic brush development apparatus.

Other objects and advantages of the present invention will become apparent to those skilled in the art from a reading of the following detailed description in connection with the accompanying drawings, in which:

FIG. 1 is a longitudinal section of a hand device for applying the devloper particles to an electrostatic latent image to be developed;

FIG. 2 is a fragmentary section through a modified form of shield as may be used with the apparatus of FIG. 1;

FIG. 3 is a side-sectional view of an embodiment of automatic apparatus for effecting development in accordance with the invention; and,

FIG. 4 is a top plan view partly in section of the embodiment of FIG. 3.

The apparatus to be described, embodying the invention in illustrative form, may be operated with a developer comprising a toner powder and a carrier material.

The carrier material consists of magnetically attractable powders such as magnetic ferrites as described in U.S. patents to J. L. Snoeck No. 2,452,529; 2,452,530 and 2,452,531 all dated October 26, 1948, iron carbonyl, alcholized iron, etc., while any of the large number of toner materials known to those skilled in the art such as those described in U.S. 2,618,551 to L. E. Walkup, U.S. 2,618,- 552 to E. N. Wise and U.S. 2,753,308 to R. B. Landrigan are suitable for mixing with the magnetic carrier to form a developer to be employed in the present invention. The ferrites developed by Snoeck described above may also be used as single component magnetic developers, i.e., the ferrite may act as both carrier and toner. (See Wilson patent, surpra.)

If alcoholized iron is used as the magnetic carrier, it is possible to develop a reversal image or a direct image by selecting a toner above or below iron in the triboelectric series. As examples, powdered shellac or rosin, each pigmented with a suitable material such as carbon black, when used with alcoholized iron give a direct image with a negative charged surface and a reversed image with a positively charged surface. Similarly, Vinsol resin (an extract from long leaf yellow pine stumps composed principally of an oxidized form of abietic acid and manufactured by Hercules Powder Company, Wilmington, Delaware) colored with a suitable dye such as nigrosine gives a direct image with a positively charged surface and a reversed image with a negatively charged surface. Alternatively, the toner may be kept the same and the magnetic carrier varied to obtain a direct or reversed reproduction of the electrostatic image. Thus, using a polystyrene resin with a suitable pigment as carbon black for the toner, one can obtain a direct image of a negatively charged surface when using PMC Z270, a powdered iron produced by Xerox Corporation, Rochester, New York, as the magnetic carrier and a reversed reproduction of the negatively charged image when using Carbonyl-L, a powdered iron made by the reduction of an iron carbonyl by Antara Chemicals Division of General Aniline and Film Corporation, New York, New York, as the magnetic carrier.

Referring now to the drawings, there is illustrated in FIG. 1 a magnetic brush, designated 10, for developing electrostatic latent images in accordance with the inven tion. The brush is comprised of a bar magnet 11 surrounded by a box-like shield 13 that is non-magnetic and yet electrically conductive. The shield may be of metal or the like and on which there is a terminal 14 for connecting a lead wire 15 thereto from a source of potential 16 via a switch 13.

In order to vary the magnitude of the potential from the source, there is provided a potentiometer 17 and for connecting the brush operatively to ground or bias potential, there is provided a double throw switch 18. By means of a selector switch 22, either polarity of potential may selectively be applied.

By having the option of electrically connecting the brush as above, versatility of the development is enhanced for developing various forms of copy by mere optional selection of switching. Assume, as an example, that an electrically insulating layer 41 bears a positively charged electrostatic image and that the switches and potentiometer are adjusted to apply a negative voltage with respect to backing plate to the shield 13. As the magnetic brush is contacted across the surface of the layer, a unidirectional electric field appears between the magnetic brush and the backing plate due to the applied biasing voltage. This unidirectional field is an addition to and enhances the electric field created by the electrostatic image such that the negatively charged developer particles 21 are more easily detached from the brush to deposit in the positive image areas. The visual powder image thus produced tends to have relatively high contrast characteristics increasing with increasing values of bias potential.

If the bias polarity i reversed and an increasing voltage positive with respect to the backing plate is applied to the shield 13, a value is reached where the positively charged areas of layer 41 are no longer developed. Minimum background deposition occurs with a positive potential of magnitude about equal to the residual background potential. On increasing the positive magnitude, the negatively charged developer particles are deposited upon the background areas to produce a reverse image. The contrast of the reverse image increases and the amount of deposit in the charged areas of the layer decreases as the positive voltage is increased in magnitude.

Thus by seting switch 18 to the bias position, a reversal reproduction, controlled background development, or image enhancement may be effected by applying the appropriate magnitude and polarity of bias potential to the shield. Connecting potentials on the order of back ground potential and of the same polarity, will effect background suppression, while potentials of the same polarity and the same or greater magnitude as the image potentials result in a reversal reproduction.

Because a grounded magnetic member has been found beneficial for developing electrostatic images comprised of wide solid areas, this result can be easily achieved simply by setting switch 18 to the ground terminal. In this position the magnet, itself, through the carrier particles and the shield constitutes a built-in conductive electrode offering a capacitance coupled to ground competing with the surface of the plate. This has been found to result in excellent development coverage of solid image areas.

When operating the brush by hand, the operator holds the brush by means of a handle 19 secured to the side of the brush and electrically insulated therefrom by an insulating Washer 20 to protect the operator from connected potentials. Developer 21, which may be of any of the above-described types, is retained by the magnetic field created by the magnet 11 along the underside of the conductive shield 13. Development is effected by moving the brush suppotring a quantity of developer particles 21 across the surface of an image-bearing member such as a xerographic plate on which an electrostatic latent image has previously been formed.

Extending around the shield is a horizontal flange 27 of non-magnetic material that extends sufliciently far from the magnetic field such that the magnetic field at the extremities of the flange is too weak to retain developer particles. This serves a useful function and eliminates the almost impossible requirement of wiping or scraping used developer from the brush to clean the member. Instead, removal of used developer is effected by merely raising the magnet above the flange causing the magnetic particles to drop off because of inability to creep over the flange. To remove or insert the magnet from the shield, there is provided an electrically insulating rod 25 attached to the magnet and which includes a knurled knob 26 for hand gripping.

Referring now to FIG. 2, there is illustrated an alternate embodiment of shield for the magnetic member of FIG. 1. In this embodiment, the interior base layer of the shield, designated 28, is comprised of a non-magnetic and electrically insulating material such as a plastic, glass, glass-fiber, plastic laminate, etc. In contact with layer 28 is a layer 29 which may comprise the exterior of the shield and consists of a non'magnetic electrically conductive material such as metal, conductive paint or other similar electrically conductive coating.

With this embodiment, the conductive layer 29 can be energized with a reference potential, which includes ground or bias, in a similar manner as described above in connection with FIG. 1. In addition, this embodiment further enhances the developing properties of the brush by extending the range of image forms with which the brush has maximum utility.

By the term non-magnetic as used herein is meant a material which does not interfere with the liens of force of a magnetic field, i.e., a material having a permeability not substantially different from that of free space. Magnetic brush development is truly a specie of cascade development. As used in the instant invention, it dilfers primarily in that a magnet is used to effect adherence to the presenting surface which carries the carrier-toner mixture over the surface of a xerographic plate, rather than the force of gravity. The carrier in magnetic brush development, therefore, is magnetic and generally ferromagnetic. As such, this magnetic carrier is, itself, frequently conductive and hence the mixture of a carrier and toner is frequently somewhat conductive, rather than purely insulating. With the development unit grounded through the apparatus in which it is employed, the mass of developer, i.e., toner-carrier mixture, acts like a grounded conductor and tends to establish strong and uniform fields above the electrostatic image as it passes by. The deposition of toner is thus uniform over large image areas. Unless a xerographic plate has been exposed so as to reduce the non-image areas completely to zero, the result will be a black-on-gray reproduction of the original, rather than a black-on-white as desired. Regulating exposure to this extent, particularly when the copy to be reproduced consists of variable quality subjects, is very difficult.

It has been found that using a non-magnetic shield which is also electrically insulating is one way to eliminate this problem. In effect, such a non-magnetic electrically insulating shield extending over large areas of the image-bearing surface causes the magnetic brush to assume the average potential it sees on the surface hearing the electrostatic image, so that the brush will float to that potential approximating or slightly above the background potential. This enables magnetic brush development to be used with an exposure latitude comparable to that obtained with cascade development without application of an external bias potential.

Therefore, in accordance with the shield embodiment of FIG. 2, there is optionally available the effects of a floating shield by merely positioning the control of switch 18 at a non-connected position. Optionally, in order to minimize shock hazard as would be associated with a conductive exterior, there may also be included overlying the conductive layer 29 another relatively insulating layer 30 (shown dashed) preferably of thin section and which may be any of the electrically insulating materials mentioned above.

Therefore, this embodiment enables not only the biasing benefit afforded by employment of a conductive shield but in addition, utility of the magnetic brush element is enhanced by having selectively available the benefit of an insulating material interposed between an image-bearing plate and the magnet.

Referring now to FIGS. 3 and 4, there is illustrated an embodiment of automatic apparatus operable in accordance with the invention. The particular embodiment is to be described as operable with a rotating shield member relative to a stationary magnet which may be a permanent magnet or an electromagnet of any suitable geometrical configuration as is known in the art. It is to be understood and to be considered as part of this invention that alternatively, a stationary shield of the type described above could be employed with a rotating magnet or either one or the other or both may move relative to the other.

As illustrated, there is shown a magnetic brush device, designated 35, for developing an electrostatic image on an image-bearing plate 40 which may be a xerographic plate having a photoconductive surface on a conductive backing. The surface of the plate contains an electrostatic latent image prepared by any of the means known to those skilled in the art as, for example, those set forth in US. 2,297,691 to Chester F. Carlson. The plate is adapted to be continuously advanced along a predetermined path to pass through the usual xerographic processing stations including sensitizing, exposure and fixing as is known in the art but not illustrated in the drawings. The plate is shown to be constructed of flexible material issuing from a supply reel 41 from which it is continuously drawn onto a take-up reel 42 being driven by motor 43. Suitable means such as a slip-clutch or the like (not shown), may be provided between the motor and the take-up reel in order to maintain the velocity of the plate substantially constant. As the plate advances to the take-up reel, it is supported on its rear or lower sur face by means of support platen 45 that is suitably grounded.

The image-bearing member or plate with which this invention is used may comprise any of a variety of forms known in the art. Thus, at least the surface layer supporting the image may be comprised of an insulating material, such as polystyrene capable of retaining a charge pattern for a reasonable period of time, or it may comprise a photoconductive insulating material such as vitreous selenium, a dispersion of zinc oxide in an insulating binder, or various other materials known in the art of xerography. When light sensitive layers bearing charge patterns are developed, then the developer should be presented in the absence of activating radiation. The backing substrate may comprise any suitable mechanical support, whether rigid or flexible such as metal, conductive paper, or the like. Optionally, the substrate may be dispensed with where the insulating layer, itself, has adequate strength.

The developing apparatus 35 is supported adjacent the path of movement of the xerographic plate to present developing particles to the plate surface during the course of plate movement. As illustrated, the developing apparatus is comprised of an elongated permanent magnet extending widthwise of the plate and secured to station ary rod supports 51 and 52, in turn secured to stationary support blocks 53 and 54.

Mounted as to substantially enclose the magnetic member and for rotation thereabout is a cylindrical shield 55 which is non-magnetic and containing an electrically conductive material similar in construction to the shield of either FIG. 1 or FIG. 2.

The shield 55 has a cross sectional diameter sufficiently large to encompass the magnetic member while the ends are of smaller diameter to form sleeve sections 56 and 57 having electrically insulating bushings 62 and 63 for mounting on the outer race of ball-bearings 58 and 59, respectively, which in turn pressfit onto rods 51 and 52, respectively. For the purpose of mounting or removing the shield onto the bearings, the shield is formed of two semi-cylindrical mating sections connected by means of two resilient spring hasps and 61. By drawing at the opposite sections, the individual sections are separable to enable convenient removal of the shield from the bearings and likewise they may be joined in assembled relation -by pressing them together when appropriately arranged. As an alternative, the sleeve can be mounted on one end instead of two. This latter method permits an integral shield which can be slipped onto the single mount.

Rotation of the shield is eifected by means of a motor 43, the output shaft of which securely supports a pulley driving a timing belt 71 connected to a toothed or ribbed portion 72 on sleeve 57. As the motor operates, there is continuous rotation of the shield during which it is the purpose of the exterior surface of the shield to carry with it a quantity of developing material for presentation to the surface of the image-bearing member 40. The rotational speed is dependent on several factors including developer being employed, plate being developed, etc. Speeds on the order of approximately /2 to 2 /2 inches per second are usually suitable with up to about 1 /2 inches per second being preferred.

embodiment as viewed in FIG. 3.

Thus the outer or exterior surface of shield 55 is not smooth but rather is roughened in a random or regular pattern such that the surface roughness serves to hold magnetic developer 21 in place and prevent its sliding along the surface as the shield rotates upwardly in this The texture of the sleeve surface may be inherently rough as by using a uniform checkerboard rough pattern on the backside. Otherwise, the texture may be formed rough as by molding a plastic shield against a wire screen and removing the wire screen once the shield material has hardened. Sandblasting has also been found a convenient method of forming a roughened surface as long as provisions are taken to provide for uniform roughness along the top surface and such roughness provides a sufiicient friction coefficient to carry developer particles during rotation of the shield. Thus, uniformity in roughness is desired so that uniformity of developer presentation results, thereby constantly supplying during operation a uniform supply of developer particles to the surface of the image-bearing member. Therefore, any known means for accomplishing uniform roughness of the shield surface is intended to be encompassed within this invention.

For the embodiment illustrated, the shield 55 is mounted to rotate about its axis in the direction shown by the arrow and about magnet Stl. The magnet 54), which may be of alnico, remains stationary with one of its poles (shown as the north pole) as close to the inner surface of shield 55 as possible to produce a magnetic field thereabout while the south magnetic pole is spaced from the inner surface of the shield to produce a weakened or no magnetic field at the shield surface. By this arrangement, the field strength generated by the pole close to the inner surface and to the left of line AA assures that the magnetic field strength to the left of the line is sufficient to cause developer to cling to the surface while to the right of the line A-A, there is insufficient field strength to retain developer on the shield surface. With this arrangement, the developing material 21, which is contained in a trough 73, is magnetically picked up by the rotating shield as the periphery of the shield moves over the trough. Thereafter, the developer particles are arranged into a brush-like array and carried upwardly or downwardly, as the case may be, to be brushed across the electrostatic latent image on surface 40, shown moving in a direction opposite to the direction of shield rotation. This then causes the image to be developed, after which the developing particles still remaining on the shield surface drop off as the surface continues to rotate beyond the magnetic field. By this means, there is produced a continuous cycle of attracting developer, brushing the developer across the image, and then returning the developer to its source of supply. It has also been noticed that the brush apparently varies during rotation of the shield helping to create a changing mix in the brush for development. With the continuous cycle of operation, developer pick up and drop off to the trough causes a continuous mixing of the developer particles and maintains uniform toner concentration.

In order to replenish the developer with toner consumed in developing, a toner supply 74 is maintained in toner dispenser 75 which dispenses a controlled quantity of toner into the trough. As the shield rotates, the developer in the trough is constantly agitated and mixed such that the brush continually presents replenished developer to the electrostatic image to assure uniform development density of each succeeding image.

If it is desired to change the composition of developer, the apparatus can be simply adapted by merely separating the two semi-cylindrical sections of the shield maintained together by spring hasps 60 and 61. The trough as shown is conveniently provided with a hinged bottom wall 80 supported by a hinge 81 and a hasp 82 such that developer contained therein may be easily and simply dumped by opening the hasp. Subsequently, a developer of a different type may be substituted while a new shield is mounted about the magnetic member 50. Optionally, dispenser may also have means provided for conveniently dumping the toner 74 and substituting a toner of a different composition assuming, of course, that developer is employed including toner as a separate component. Alternately, by supporting the magnet removably independent of the shield, removing the magnet will cause developer drop off to the trough. In this latter arrangement, there may be included an annular flange on the shield at the withdrawal end to effect developer drop off in the manner of FIG. 1.

In order to apply the electrical connections to the shield as in the manner of FIGS. 1 and 2, sleeve 56 contains a conductive ring-collar 76 integral therewith and to which a sliding contact is made by a shoe 77. The shoe is connected via a lead wire 15 through a switch 18 and to a potentiometer 17 to a potential source 16. Thereby, whether the shield 55 is electrically conductive in the manner of FIG. 1 or further, is formed of the insulating layer or layers of FIG. 2, ground, electrical bias, or an electrical isolation can be achieved as described above in accordance with the results sought to be achieved.

There has thus been described an improved and novel magnetic brush developing apparatus for applying electroscopic developer powder to an electrostatic image. By means of the invention, there is accorded greater facility than heretofore in extending the useful range of magnetic brush development by controlling the electrical properties of the shield supported about the magnetic member. The apparatus provides simple and convenient means for selectively altering the intrinsic electrical properties of the magnetic brush in accordance with the properties of the electrostatic image to be developed.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention can be made Without departing from the scope thereof, it is intended that all matter contained in the drawings and specification shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. Apparatus for developing electrostatic latent images previously formed on the surface of a support member, said apparatus comprising in combination:

(a) a magnetic field producing means;

(b) a shield at least partially surrounding said field producing means Within the effective magnetic field thereof and capable of retaining magnetic powder developer particles attracted thereto by the field producing means for presentation to the surface of an image support member, said shield including:

(1) a first layer of electrically insulating material in close spaced relation to the magnetic field producing means; and,

(2) a second layer of electrically conductive ma terial overlying said first recited layer;

(0) a terminal for connecting a reference potential to said electrically conductive layer;

((1) switch means including bias potential, neutral, and ground source contacts to selectively connect said terminal; and,

(e) means to support a quantity of magnetic powder developer particles for supply to said shield.

2. Apparatus for developing electrostatic latent images previously formed on the surface of a support member, the apparatus comprising in combination:

(a) a magnetic field producing means;

(b) a shield at least partially surrounding said field producing means within the effective magnetic field thereof and capable of retaining magnetic powder developer particles attracted thereto by the field producing means for presentation to the surface of an image bearing support, said shield including:

(1) a first layer of electrically insulating material in close spaced relation to the magnetic field producing means; (2) a second layer of electrically conductive material overlying said first recited layer; and, (3) a third layer of electrically insulating material overlying said conductive layer and capable of retaining magnetic powder developer particles attracted thereto by the field producing means for presentation to the surface of an image support member; (c) a terminal for connecting a reference potential to said electrically conductive layer; (d) switch means including bias potential, neutral, and ground source contacts to selectively connect said terminal; and,

(e) means to support a quantity of magnetic powder developer particles for supply to said shield.

References Cited by the Examiner UNITED STATES PATENTS Jobke 209-223 Simmons et a1.

Wilson 118-637 X Giaimo 117-175 Buus et a1. 209219 Hall et al. 117-17.5 X

Bliss 118637 Sugarman 951.7 Greig 118-637 X CHARLES A. WILLMUT'H, Primary Examiner.

15 WILLIAM D. MARTIN, WILLIAM B. PENN,

Examiners. 

1. APPARATUS FOR DEVELOPING ELECTROSTATIC LATENT IMAGES PREVIOUSLY FORMED ON THE SURFACE OF A SUPPORT MEMBER, SAID APPARATUS COMPRISING IN COMBINATION: (A) A MAGNETIC FIELD PRODUCING MEANS; (B) A SHIELD AT LEAST PARTIALLY SURROUNDING SAID FIELD PRODUCING MEANS WITH THE EFFECTIVE MAGNETIC FIELD THEREOF AND CAPABLE OF RETAINING MAGNETIC POWDER DEVELOPER PARTICLES ATTRACTED THERETO BY THE FIELD PRODUCING MEANS FOR PRESENTATION TO THE SURFACE OF AN IMAGE SUPPORT MEMBER, SAID SHEILD INCLUDING: (1) A FIRST LAYER OF ELECTRICALLY INSULATING MATERIAL IN CLOSE SPACED RELATION TO THE MAGNETIC FIELD PRODUCING MEANS; AND, (2) A SECOND LAYER OF ELECTRICALLY CONDUCTIVE MATERIAL OVERLYING SAID FIRST RECITED LAYER; (C) A TERMINAL FOR CONNECTING A REFERENCE POTENTIAL TO SAID ELECTRICALLY CONDUCTIVE LAYER; (D) SWITCH MEANS INCLUDING BIAS POTENTIAL, NEUTRAL, AND GROUND SOURCE CONTACTS TO SELECTIVELY CONNECT SAID TERMINAL; AND (E) MEANS TO SUPPORT A QUANTITY OF MAGNETIC POWDER DEVELOPER PARTICLES FOR SUPPLY TO SAID SHIELD. 